In a typical in situ leach operation, wells are completed into a mineral or metal value bearing (e.g. uranium) formation and a lixiviant is flowed between injection and production wells. The uranium and/or related values are dissolved into the lixiviant and are produced therewith to the surface where the pregnant lixiviant is treated to recover the desired values.
In developing such a leach operation, several factors must be considered in order for the operation to be a commercial success. One of the most important but often overlooked consideration is the manner in which the injection and/or production wells are completed into the ore bearing formation. Improperly completed wells may lead to an early abandonment of an operation which might have otherwise had a long production life. For example, in many leach operations, a large volume of sand or other particulate material becomes entrained with the lixiviant as it passes through the formation and is produced therewith. As recognized in the art, this entrained sand is highly detrimental to production equipment such as down hole pumps and often leads to high maintenance costs and costly delays in production. Also, this sand has to be separated from the lixiviant and disposed at the surface of thereby substantially increasing the overall costs of the leach operation.
It has long been known that the problems of sand production can be alleviated by "gravel packing" the production interval of the well. There are several known types of gravel pack completion techniques. One involves casing the well through the formation, setting a screen or slotted liner adjacent the production formation and then placing gravel behind the casing (see U.S. Pat. No. 3,353,599). Another involves drilling a hole through the formation, casing the hole down to the formation, underreaming the formation and then either setting the screen and placing gravel around the screen (see U.S. Pat. No. 4,192,375) or first placing the gravel and then washing the screen into place (see U.S. Pat. No. 3,362,475). However, in all of these techniques, the well is drilled all the way through the production formation before the casing is set and cemented in place. This exposes the production formation to the cement and cement water during the casing operation which may seriously damage the permeability of the formation which, in turn, adversely affects the injection and/or production flowrates into or out of the formation during a leach operation.
Further, in a leach operation, it is extremely important to limit as close as possible the vertical injection and production intervals in the injection and production wells, respectively, to the actual ore bearing zone. This insures the most desirable flowpath between the wells for the lixiviant thereby preventing undue migration and/or loss of the lixiviant as it flows through the one zone. Therefore, it is vital that the screen in a gravel pack completion be adjacent these respective intervals in the respective wells.